The invention relates to a self-supporting emission electrode in an electrostatic dust separator having laminar collecting electrodes which guide the gas flow, which emission electrode, extending parallel to the collecting electrodes and suspended on a supporting lug, comprises a single-piece metal sheet symmetrically folded to form a support section, which imparts mechanical strength, and has emission arms, disposed in at least two rows and extending along the central plane between the collecting electrodes, which have emission tips extending in the plane of the emission arms or directed on both sides towards the adjacent collecting electrodes. Furthermore, the invention relates to a method of producing the emission electrode.
In an electrostatic dust separator, termed an electrostatic filter for short, the gas to be purified is passed through many parallel channels of a housing. The channels are formed by a plurality of collecting electrodes which are arranged in rows behind one another and which may reach linear dimensions of 15 m and over. Disposed centrally and longitudinally between the collecting electrodes are the emission electrodes.
Whereas the collecting electrodes of a dust separator are, as a rule, earthed, the emission electrodes are at a high negative direct voltage which may be in the region of 100 kV. An electric force field is produced between the two electrodes. The electric force concentration at the emission electrode has to be great enough to produce a glow or corona discharge, which manifests itself as an intense, bluish glow. The emerging electrons ionize the air and other gases forming the atmosphere. The negative and positive ions produced during the ionization migrate to the electrodes of opposite polarity.
The migrating ions collide for their part with dust particles suspended in the gas flow, adhere to them and consequently impart an electric charge to them. Under the action of the electric field, the charged dust particles are attracted by the electrodes of opposite polarity. The overwhelming majority of the dust particles are negatively charged and they deposit at the positive collecting electrode. Only 1-3% of the dust particles are positively charged and deposit at the emission electrode having negative potential.
The dust particles do not all, however, give up their charge immediately to the electrode concerned and form, also as a consequence of adhesion and cohesion, loosely coherent layers of solid material.
When the dust layer has reached a thickness of 1-2 cm, it has to be detached from the electrode. This periodic cleaning is carried out in dry filters by tapping or shaking devices, and in wet filters by washing devices. In practice, tapping is carried out, for example, 1-8 times per hour.
For the efficiency of electrostatic filters, the amount of gas flowing through, the physical nature of the carrier gas, its humidity and temperature, the electric resistance and the behavior of the dust in the electric field are of importance. Finally, the particle composition and chemical analysis of the dust, the characteristics of the operative electric field, the gas velocity, the whirling up again of the dust on tapping, the gas composition, and the current and the voltage concomitantly determine the migration velocity of the electrically charged particles.
EP-A2 0,287,137 describes two variants of emission electrodes made of sheet-metal strips of continuously identical width.
According to a first variant, the emission electrode is shaped to form an approximately elliptical tubular cross section, with overlapping longitudinal edges which are joined to one another. Individually bent out of the tubular cross section are approximately triangular lugs. The lugs form on either side of the elliptical tubular cross section, in line with its main axis, outwardly pointing vanes with alternatingly bent emission tips.
According to a second variant, instead of an elliptical tubular cross section, two wide edge strips are bent of a narrow central strip at an angle in opposite directions. The longitudinal edges of the edge strips are flanged over in the same direction as the respective angling in a manner such that an essentially stretched Z-shaped cross section is produced. Approximately triangular lugs which are not situated on the central plane between the two parallel limbs are individually bent out of the edge strips, as in the first variant.
This embodiment of an emission electrode has, in relation to the configuration, the disadvantage that the bent lugs are restricted to a length which is below the major axis of the ellipse or the width of an edge strip. Furthermore, the production appears to be comparatively complex.
Furthermore, British Patent Specification 1,575,404 discloses an emission electrode for electrostatic separation which comprises a long, suspended support section and shoulder-forming elements, joined to the support section, for forming a corona. The support section comprises a metal strip and has a stiffener extending centrally in the longitudinal direction. The longitudinally central stiffener has open parts of channel-shaped design on either side, for example in the form of a longitudinally extending corrugated fold. This embodiment has the disadvantage that it is not capable of imparting the stiffness of a conventional tubular support section. Furthermore, only single-piece embodiments which have emission tips forming sawtooth-like shoulders which are disposed near the support section are shown. Since they are disposed in the region of the support section, the emission tips of a plurality of emission electrodes are not ideally distributed. Since the metal sheets cannot be of an arbitrarily wide construction, a two-part embodiment of the emission electrode having emission arms individually attached to a parent body is formed to achieve a better distribution of the emission tips (FIGS. 7 and 8).